Recited in Patent Literature 1 as one example of systems which measure a distance by a propagation time of ultrasonic is an ultrasonic object measuring device. The ultrasonic object measuring device recited in Patent Literature 1 subjects a tone burst wave generated from a tone burst wave generator to frequency diffusion modulation by a pseudo noise signal from a pseudo noise signal generator by means of a frequency diffusion modulator and transmits the obtained signal.
Cross-correlation is obtained by a cross-correlation unit between a signal obtained by demodulating a received reflected wave by a frequency diffusion signal demodulator and a pseudo noise signal used in frequency diffusion. Furthermore, from the correlation level obtained by a correlation level detector 9, reception/non-reception of a reflected signal on an object is determined to measure a distance.
In addition, a transmission tone burst wave is sent out with a frequency diffusion modulated signal obtained by sequentially switching and distinguishing a plurality of pseudo noise signals to distinguish and determine a plurality of tone burst waves.
Patent Literature 1: Japanese Patent Laying-Open No. H07-104063
Non-Patent Literature 1: Hiroshi Kashiwagi, “M-sequence and Its Applications” (Shokodo, Mar. 25, 1996).
Since for an ultrasonic signal transmitted from an ultrasonic emission source, a frequency higher than an audible band which frequency is audible to no person is used, at least not lower than 20 kHz is required. Known as a unit for generating a signal of this frequency band with enough sound pressure is a so-called speaker which electromagnetically vibrates and drives a small-sized diaphragm with high rigidity. Because of difficulty of down-sizing and large power consumption due to current-drive, however, it is difficult to mount a speaker on a small-sized moving body. As a result, a piezo-electric element which is driven by voltage is widely used as an ultrasonic emission source.
Although the piezo-electric element has small power consumption because it is driven by voltage, it is often used in combination with a resonator whose acoustic impedance is low in order to ensure enough sound pressure. When a resonation phenomenon is used, however, while ultrasonic can be emitted with fixed phase, frequency and gain, a transmission gain at other frequencies is considerably low, so that it is difficult to use a modulation system having wide band frequency characteristics such as frequency diffusion modulation.
In addition, since also in a single piezo-electric element, a mechanical Q is high to have residual oscillation remain for long, it is difficult to emit ultrasonic precisely following a modulated wave irrespective of a modulation system.
Since when a transmission source having heavy residual oscillation is used in a transmitter for such ultrasonic or when a transmission source is shorter of band than a frequency band necessary for transmission and reception of ultrasonic subjected to frequency diffusion modulation, it is difficult to transmit an ideal modulated wave, it is accordingly difficult to suppress a subsidiary correlation peak caused by non-pseudo noise.